Pyruvic acid is an important intermediate in the in vivo metabolism and a starting material for synthesizing various physiologically active substances. For example, L-tryptophan is obtained by enzymatic reaction of triptophanase on indole, pyruvic acid, and ammonia.
Conventional processes for preparing pyruvic acid include a process comprising reacting sodium cyanide and acetyl chloride to synthesize acetyl cyanide and hydrolyzing the acetyl cyanide, and a process comprising reacting tartaric acid and potassium hydrogensulfate. These processes not only start with expensive raw materials but attain low yields and have therefore been regarded not advantageous.
It has been proposed to oxidatively dehydrogenate a lactic ester in a gaseous phase to prepare pyruvic acid as disclosed in JP-B-57-24336 and JP-B-56-19854 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-A-54-122222 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This process also uses expensive lactic acid as a raw material and requires conversion of lactic acid to its ester. In addition, the reaction product is obtained in the form of a pyruvic ester, it should be subjected to hydrolysis to obtain pyruvic acid. Therefore, the process is not always industrially advantageous.
Processes for preparing pyruvic acid by liquid phase oxidation include a process using hydroxyacetone as a starting material as disclosed in JP-A-54-39016, JP-A-54-76524, and JP-A-54-132523, a process starting with lactic ester as disclosed in JP-A-58-62136, and a process starting with lactic acid as disclosed in JP-A-54-138514 and JP-A-55-33418. However, hydroxyacetone is difficult to obtain and expensive. The process using a lactic ester attains low yields and is attended by the safety problem. The process using lactic acid is industrially unsatisfactory because of expensiveness of lactic acid.
JP-B-51-28614 discloses a process in which propylene glycol is contacted with molecular oxygen in an alkaline aqueous solution in the presence of a platinum element as a catalyst as disclosed in JP-B-51-28614. This process, however, chiefly aims at preparation of lactic acid, only obtaining low selectivity to pyruvic acid. Although it is suggested in the publication that reaction be conducted at relatively low temperatures for the synthesis of pyruvic acid, the selectivity to pyruvic acid reached in the working example where the reaction was conducted at 30.degree. C. was only 51%.
JP-A-54-132519 also relates to oxidation of glycol, claiming that a combination of a platinum catalyst and lead shows an increased catalytic activity. The publication describes that oxidation of propylene glycol in the presence of such a catalyst gives lactic acid in high yields, but there is no description about the preparation of pyruvic acid. Although this process employs a relatively low reaction temperature (i.e., 40.degree. to 50.degree. C.), the yield of pyruvic acid was proved low by the inventors' study. The pH of the reaction system is recommended to be maintained at 8 to 11 in the publication, but the inventors also revealed that it is still difficult to obtain pyruvic acid in high yields even with the pH controlled.
On the other hand, nothing is known about purification of pyruvates. There is only found a proposal of using isopropanol in isolation of pyruvates as disclosed in JP-A-55-98132.
A pyruvate is poor in heat stability and, when concentrated, liable to form a dimer even in a low temperature. Also during the reaction, for example, the liquid phase oxidation of propylene glycol, there are produced various impurities, such as an acetate formed by decarboxylation of a pyruvate, a pyruvate dimer, and a lactate as an intermediate. The above-described technique of JP-A-55-98132 fails to substantially remove these impurities.